Drilling muds are usually water-based, but they can be based on other liquids such as synthetic oils. Additives are mixed with the liquid base. Common additives to water-based drilling muds include solids such as barite, chalk (calcium carbonate) and haematite. It is required that these added solids be homogeneously mixed with the liquid base, and that the homogeneity be maintained.
The physical and chemical characteristics of drilling mud also varies during the process of drilling. Depending on the geology at the depth of the drill bit, it may be necessary for the driller to actively vary any one or more of the density, viscosity, pH, or other chemical or physical property of the drilling mud. In the oil industry, when drilling a borehole, the drilling muds used during the life-cycle of a single borehole could begin with water, then move to a water based mud, then move from the water-based mud to a synthetic oil based mud. These drilling muds have a complex range of physical characteristics and the characteristics required at any particular stage of the drilling process vary during the drilling life-cycle. Physical or chemical characteristics of the mud may also vary depending on events which are not under the control of the driller. The invasion of petroleum products into the bore hole is such an event, and will cause a “kick” or impulse change in the characteristics of the drilling mud, causing sudden variations in, for example, the density and/or viscosity of the mud.
It is accordingly important for the driller to monitor volumetric flows of the drilling mud.